Cardiac output, the average blood flow through the heart, is an important diagnostic measurement, especially for patients in coronary care units (CCU). Since the introduction of the Swan-Ganz catheter in the early 1970's, cardiac output has been routinely measured by thermodilution. The Swan-Ganz catheter is inserted into the femoral or subclavian vein and advanced so that a fluid port along the catheter is positioned in the right atrium (RA) and the tip of the catheter is in the pulmonary artery (PA). Generally, a 10 cc bolus of iced saline is injected through the fluid port along the catheter into the right atrium. A thermistor, or other temperature-sensing device, is located along the catheter within the pulmonary artery. The blood temperature is then monitored at the is thermistor as the blood flows through the pulmonary artery. The average cardiac output can be computed from the relative temperature fluctuation within the pulmonary artery. Specifically, the average cardiac output is an inverse function of the cooling as measured in the pulmonary artery.
There are two noted disadvantages to measuring cardiac output by bolus thermodilution. First, to avoid an undue fluid load on the patient, readings can only be taken periodically. Second, these periodic measurements are labor intensive as they require a nurse or other clinician to manually intervene to make each fluid injection.
Various methods proposed for the continuous method of cardiac output have typically involved substituting heat for chilled saline. U.S. Pat. No. 4,576,182, (the “'182 patent”) titled METHOD AND APPARATUS FOR MEASURING LIQUID FLOW, issued to Richard A. Normann, describes an electrically resistive heating element wound around the outer surface of a catheter to introduce heat for thermodilution measurements. One advantage of such heating is that it can be electronically controlled, thereby allowing easier implementation of continuous monitoring systems. However, blood does not tolerate heating as well as it tolerates cooling. Consequently, the heating power that can be safely delivered to the blood is much less than the cooling power that is delivered by a 10 cc injection of iced saline. This lower heating power produces a much smaller heat pulse in the pulmonary artery. For example, an iced saline injection can produce a thermal fluctuation as large as 1–2° C., whereas the maximum thermal fluctuation that can be produced by heating, without damage to blood components, is between 10–20 m° C.
With such a small temperature fluctuation resulting from the heat pulses, thermal noise can make accurate thermal measurements of cardiac output difficult. FIG. 1 shows a thermal signal recorded in the pulmonary artery of a pig that was attached to a respirator set at a rate of approximately 12 breaths a minute. The approximate 20 m° C. regular fluctuations in FIG. 1 are the result of this respiration. The thermal noise generated by the respiration can significantly reduce the ability to detect a thermal pulse generated by a heating element on a catheter. Such noise in the thermal pulse recorded in the pulmonary artery can be reduced by averaging consecutive pulses. The accuracy of the thermal measurements increases with the number of pulses that are averaged, but as the number of pulses being averaged increases, the time required to generate an accurate estimate also increases to a length that reduces the usefulness of the procedure.
U.S. Pat. No. 4,507,974 (the '974 patent) titled METHOD AND APPARATUS FOR MEASURING FLOW, issued to Mark L. Yelderman, describes the application of a pseudo-random sequence of heat pulses to the measurement of cardiac output. The output of the downstream temperature sensor is cross-correlated with the pseudo-random sequence. The correlation materially reduces the effects of high-frequency thermal noise and a reasonable estimate of the thermal pulse signal can be obtained in five to eight minutes. However, low-frequency noise, primarily resulting from the effects of respiration is not reduced by this method.
U.S. Pat. No. 5,146,414 (the “'414 patent”), for a METHOD AND APPARATUS FOR CONTINUOUSLY MEASURING VOLUMETRIC FLOW, issued to Russell McKowan et al., describes a complex system for reducing low-frequency noise in thermal pulse measurements by matching the length of the pseudo-random sequence to a multiple of the respiration period. A reduction of the baseline artifact is obtained by estimating the artifact and subtracting it from the detected signal. This method works well when a patient is on a ventilator that imposes a uniform respiration rate. However, it is not easily applied to the irregular patterns that characterize normal respiration. Moreover, the baseline estimation described in the '414 patent is based on polynomial fitting to discrete points separated by 1–2 minutes, which provides only a rough estimate of very-low-frequency base line fluctuations and it thus introduces a significant delay before the thermal signal can be processed.